1. Field of the Invention
The present invention relates to solder paste printing method and apparatus for printing solder paste on a board formed thereon with wiring patterns.
2. Description of the Related Art
Hitherto, soft soldering is usually employed for fixedly mounting electronic components on a printed circuit board (it will be referred to as a PCB). A description of an example of the method of mounting electronic components by the use of a soft solder is provided hereinbelow with reference to FIG. 1. Here, the description of a case where soldering is appliedby the known reflowing technique on both surfaces of a PCB, respectively, will be provided.
First, a metal mask provided with apertures at positions corresponding to land portions of a PCB is used for printing solder paste on the land portions (Step 101). Subsequently, electronic components such as chips, QFP (Quad Flat Package), SOP (Small Outline Package) and so on are mounted on the PCB, so that the electric terminals and leads of these electronic components are mounted onto the printed solder paste (Step 102). Thereafter, the PCB mounting thereon the electronic components is urged to pass through a high-temperature reflowing furnace so as to fuse the solder paste thereby soldering the electrodes of the electronic components to the land portions of the PCB (Step 103).
The described process permits completion of the mounting of the electronic components onto one of both surfaces of the PCB. Therefore, the PCB is subsequently reversed so that the other surface mounting thereon no electronic components is held upward (Step 104).
Subsequently, similarly to the described Steps 101 and 102, the printing of the solder paste (Step 105) and mounting of the electronic components (Step 106) are carried out. Thereafter, the components having their electric leads are inserted in the through-holes (Step 107). Then, similarly to the Step 103, the PCB is urged to pass through the furnace so as to complete the soldering of the components (Step 108).
Finally, some electronic components that are not able to withstand a high temperature in the reflowing furnace are subjected to the process for manually soldering these components to thereby complete mounting of the electronic components onto the PCB (Step 109).
In the above-described mounting method of the electronic components according to the known technology, solder paste containing therein solder of tin and lead (Sn—Pb) system is generally used. However, since the Sn—Pb system solder contains therein lead (Pb) that is a toxic heavy metal, unless electronic appliances after usage are adequately put on the discard, there has occurred such a problem that an adverse affect is provided on the global atmosphere. Taking this into consideration, in recent years, in order to solve the described problem to thereby prevent the environmental pollution beforehand, employment of a Pb-free solder containing therein no lead component has long been desired.
A tin and silver (Sn—Ag) system solder is typically known as the Pb— free solder. Since the property of the silver (Ag) is stable, when the Sn—Ag system solder is used for the mounting of the electronic components in lieu of the Sn—Pb system solder, it can ensure identical degree of reliability with the conventional mounting method. Nevertheless, compared with the fact that the melting point of the Sn—Pb system solder is at about 183° C., the melting point of the Sn—Ag system solder is at about 220° C., which is rather high. Therefore, the conventional mounting apparatus and method having employed the Sn—Pb system solder cannot be directly applied when the Sn—Ag system solder is employed.
If the Sn—Ag  system solder having the melting point of as high as 220° C. is fused in the reflowing furnace to carry out the soldering of the electronic components, the temperature of the components could occasionally be more than 240° C. Since the heatproof temperature of the general electronic components is at approximately 230° C., when the Sn—Ag system solder is employed for mounting the electronic components, such a problem must be encountered that the heatproof temperature of various sorts of electronic components should be raised.
There is another Pb-free solder different from the Sn—Ag solder having the above-mentioned high melting point, i.e., a tin-Zinc (Sn—Zn) system solder. Since the melting point of the Sn—Zn system solder is at approximately 197° C., when the Sn—Zn system solder is employed for the mounting of electronic components, the conventional equipments and electronic components can be directly employed without any change to them.
Nevertheless, when the Sn—Zn system solder is compared with the conventionally employed Sn—Pb system solder, there are problems such that the Zinc (Zn) is apt to be oxidized, and the wettability of the Sn—Zn system solder is rather poor. Accordingly, when the mounting of the electronic components is conducted by the direct use of the conventional equipments and the conventional mounting method, it cannot be ensured that the mounting reliability is equivalent to the conventional one.
At this stage, the printing process of the above-described solder paste will be described with reference to FIGS. 2A through 2C.
First, as shown in FIG. 2A, onto board 204, printing mask 250 is positioned and mounted so that respective apertures 250a of printing mask 250 are in correspondence with respective lands 203. Subsequently, a predetermined amount of solder paste 251 is placed on printing mask 250 mounted on board 204, and as shown in FIG. 2B, squeegee 252 is used for urging solder paste 251 to perform rolling over the surface of printing mask 250 from an end of the surface to the opposite end.
While solder paste 251 is rolling over the surface of printing mask 250, it is impressed into respective apertures 250a by the aid of squeegee 252 so as to fill up apertures 250a. Then, as shown in FIG. 2C, when printing mask 250 is separated away from board 204, a predetermined amount of solder paste 251 is printed on each of lands 203 of board 204, and thus the printing process of the solder paste is terminated.
With the solder paste containing therein the conventionally employed Sn—Pb system solder, the above-described printing process for the solder paste proceeds under such atmosphere that the temperature and humidity are at about 27° C. and 60%, respectively. Thus, relatively thick moisture accordingly, if the flux component metamorphoses under a circumstance such that the solder paste is apt to be moistened, the lead (Pb) per se is a stable metal, and as a result, the Pb does not increase viscosity thereof due to reacting with the flux component for a short time. Therefore, the printing of the solder paste can be executed without causing any problem.
On the other hand, in the case of the solder paste containing therein the Sn—Zn system solder, the reaction of the flux component, which metamorphoses due to being moistened, with the zinc (Zn) that is an active metal proceeds for a short period of time, and accordingly the viscosity of the solder paste increases and is degraded. The degradation of the solder paste occurs in approximately three hours since the commencement of the printing process of the solder paste.
When the solder paste increases in its viscosity to become sticky, the rolling property thereof on the printing mask is lowered, and the solder paste is apt to be attached to the squeegee. Thus, during the impressing of the solder paste into the apertures of the printing mask by the aid of the squeegee, the solder paste fails to be sufficiently filled in the apertures and as a result, there is a possibility of causing a failure in the printing. Therefore, with the time lapse of about three hours after the commencement of the printing process of the solder paste, the solder paste needs to be replaced with fresh one.
Further, during the printing process of the solder paste, when the material of the solder, especially the zinc (Zn), is oxidized by the reaction with an oxygen contained in the atmosphere, wettability of the solder material is degraded. As a result, many solder balls are generated during the mounting process of the electronic components.